1. Field of the Invention
The present invention relates to a conductive ceramic bearing ball, a method for manufacturing the conductive ceramic bearing ball, a ball bearing containing the conductive ceramic bearing ball, a motor having a bearing member including the ball bearing, a hard disk drive including the motor for rotating a hard disk, and a polygon scanner including the motor for rotating a polygon mirror.
2. Description of the Related Art
A bearing ball is generally made of metal, such as bearing steel. However, in order to impart enhanced wear resistance, the manufacture of bearing balls from ceramic is known. Examples of ceramic for use in manufacturing bearing balls include silicon nitride ceramic, alumina ceramic and zirconia ceramic.
3. Problems Solved by the Invention
Since the above-mentioned bearing balls are composed of an insulating material, during rotation as rolling elements in a bearing, the bearing balls tend to be electrified (charged) with static electricity generated by friction. When such charge becomes excessive, during manufacture of, for example, small-diameter bearing balls, the bearing balls may adhere to apparatus (e.g., a container), or dust may adhere to the bearing balls, thereby hindering smooth progress of working.
Since bearing balls of a bearing used in precision electronic equipment, such as a hard disk drive of a computer, rotate at high speed, electrostatically induced adhesion of foreign matter, such as dust, to the bearing balls or to an inner or outer ring is highly likely to cause undesirable noise and vibration.
In application to such precision electronic equipment, a conductive ceramic bearing ball has been proposed having a microstructure composed of insulating ceramic matrix and conductive ceramic dispersed in the matrix. However, when a conductive ceramic phase is contained in a large amount to enhance electrical conductivity, the bearing ball may suffer impaired strength or wear resistance. Also, a certain type of impurity contained in bearing ball material may cause defects, such as excess noise or vibration, in application to precision electronic equipment involving high-speed rotation, such as a hard disk drive of a computer or a polygon scanner.
Furthermore, when a bearing is rotated at high speed, the temperature of the bearing rises to approximately 60xc2x0 C.-100xc2x0 C. because of self-generated heat induced by sliding. In this case, if the coefficient of thermal expansion differs greatly between the ceramic constituting the bearing balls and the steel material constituting an inner or outer ring, the rotational precision of the bearing is impaired.
It is therefore an object of the present invention to provide a conductive ceramic bearing ball which resists charging, exhibits practically sufficient strength and wear resistance, and does not differ greatly in coefficient of thermal expansion from a ferrous material constituting an inner/outer ring, to thereby maintain rotational precision of a bearing which would otherwise degrade due to an increase in temperature induced by frictional heat generation. It is also an object of the present invention to provide a ball bearing using the conductive ceramic bearing ball, a motor having a bearing using the ball bearing, a hard disk drive using the motor, and a polygon scanner using the motor.
The above objects of the present invention are achieved by providing a conductive ceramic ball comprising a mixed microstructure of a silicon nitride phase and a titanium nitride phase and having a content ratio of said silicon nitride phase to said titanium nitride phase such that the conductive ceramic bearing ball has an average coefficient of linear expansion within a temperature range of 20xc2x0 C.-100xc2x0 C. of from 2xc3x9710xe2x88x926/K to 5xc3x9710xe2x88x926/K.
Also, a ball bearing of the present invention is characterized in that a plurality of conductive ceramic bearing balls are incorporated as rolling elements between an inner ring and an outer ring made of a ferrous metal containing a predominant amount of iron. The inner and outer rings (i.e., races) of the bearing can be made of a steel material having a Ni content of not greater than 3% by weight (including 0% by weight), such as high-carbon chromium bearing steel (for example, SUJ1, SUJ2, or SUJ3 as described in JIS G 4805 (1990)) or martensitic stainless steel (for example, SUS440).